Photo-voltaic power generating means and methods

ABSTRACT

A photo-voltaic power cell based on a photoelectric semiconductor compound and the method of using and making the same. The semiconductor compound in the photo-voltaic power cell of the present invention can be electrolytically formed at a cathode in an electrolytic solution by causing discharge or decomposition of ions or molecules of a non-metallic component with deposition of the non-metallic component on the cathode and simultaneously providing ions of a metal component which discharge and combine with the non-metallic component at the cathode thereby forming the semiconductor compound film material thereon. By stoichiometrically adjusting the amounts of the components, or otherwise by introducing dopants into the desired amounts, an N-type layer can be formed and thereafter a P-type layer can be formed with a junction therebetween. The invention is effective in producing homojunction semiconductor materials and heterojunction semiconductor materials. The present invention also provides a method of using three electrodes in order to form the semiconductor compound material on one of these electrodes. Various examples are given for manufacturing different photo-voltaic cells in accordance with the present invention.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

The Government has rights in this invention pursuant to contract No.DE-AC01-76ET20218 awarded by the U.S. Department of Energy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 899,227 filed Apr. 24,1978, now abandoned, which is a continuation-in-part of the copendingapplication Ser. No. 883,150, filed on Mar. 3, 1978, now abandoned,which in turn is a continuing application of the parent application,Ser. No. 693,890, filed on June 8, 1976, now abandoned, all applicationsbeing assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to systems and methods for generating electricalpower from light radiation, and specifically from solar radiation.

2. Description of the Prior Art

The increasingly aggravated inadequacy of fossil fuels for energygeneration of all types has led to many efforts to tap alternativeenergy sources. A particularly attractive alternative source is lightradiation, and particularly solar radiation, which comprises enormousamounts of easily accessible energy and is largely untapped. Among themost important existing devices for converting solar energy intoelectricity are devices of the type developed in the space effort. Thesedevices comprise networks of smaller area thin monocrystalline layersconnected in series. These devices have relatively high efficiency interms of power generation in relation to weight. This criterion,however, is substantially inapplicable to the problem of powergeneration for normal commercial and consumer purposes, in which thecriterion of usefulness is related to economic factors, such as powergeneration per unit cost. Under this criterion of efficiency units whichare useful in the space effort are impractical. These units arecomprised of photoelectric material which is substantiallymonocrystalline and must be grown from crystalline solution, with a highfailure rate. These constraints limit such units to small dimensions andrequire many such units to provide even a minimal power source.

Devices and fabrication techniques utilizing polycrystallinesemiconductive materials have generally proven inadequate due to highproduction costs. Among the contributing factors to these high costs isthe requirement of use of structural materials of high heat resistancedue to the high temperature utilized with these fabrication techniques.Moreover, such devices generally utilize metallic internal conductors,thus further increasing costs. Also, the failure rate in fabricatingsuch devices is relatively high because of penetration by impurities, inthe course of fabrication. Further, control of the deposition ofsemiconductive material in such processes presents substantial problems.

There has been a recent attempt to form photo-voltaic power generatingmeans with a compound semiconductor material having an N-type region anda P-type region and in which the N-type and P-type regions were doped.In this case, the first semiconductor section, constituting an N-typesection, was formed by a vapor phase deposition of a metal, such acadmium with the addition of sulfur to provide a cadmium sulfide layer.The second, or P-type, semiconductor section was formed by dipping thematerial into a hot aqueous solution of CuCl which caused formation ofCu_(x) S. However, the results were poor since the cadmium sulfide wassometimes porous, giving rise to shorted junctions. In addition, a largeamount of unused cadmium was required in the deposition, therebycreating a substantially expensive photo-voltaic power generating means,oftentimes of low efficiency.

U.S. Pat. No. 3,573,177 to William McNeill describes a prior arttechnique by which polycrystalline cadmium, zinc, or cadmium-zincsulfide or selenide is formed by electrochemical deposition on a Cd orZn or Cd, Zn anode and where sulfur or selenium is provided from asolution containing S⁼ or Se⁼ ions and which polycrystalline material isusable as a semiconductor material. The concept of forming thin films ofsemiconductor materials by electrochemical techniques is relatively newand due, in part, to the teachings in the aforesaid McNeill patent. Inaccordance with the McNeill patent, electrochemical discharge of ions,such as those yielded by sulfides or selenides dissolved in anelectrolyte, occur with respect to cadmium or zinc acting as an anode inan electrolytic cell. This electrochemical discharge converts the zincor cadmium, or the alloys of these metals, to the corresponding sulfidesor sulfoselenides.

The McNeill patent has advanced the art of producing semiconductivematerials by electrochemical techniques and presents many advantages,including the ability to apply films of semiconductor materials toirregularly shaped substrates which were not thoroughly cleaned.Nevertheless, the McNeill patent suffers from many limitations in thatthe end product is not necessarily capable of functioning as a P-Nsemiconductor junction material necessary in the operation of aphoto-voltaic cell or similar diode. McNeill is essentially concernedwith the manufacture of non-junction semiconductor films, such as thosefound in the electroluminescent panels, electrosonic transducers andphotosensitive conductors.

The essence of the McNeill patent lies primarily with anodic plating,with discharge of S⁼ and Se⁼ ions. However, it is not known that theMcNeill process can be applied to e.g. discharge of Te⁼ ions which ismore ideal in the case of photo-voltaic cells. Yet it is such adischarge, forming CdTe and a ZnTe that is of prime importance for themanufacture of solar cells since cadmium telluride has a direct band-gapuniquely optimized for sunlight at 1.5 eV.

There has also been a proposed prior art technique for electrochemicallyprecipitating metals at a cathode for producing a selenium rectifier.This technique is reported in an article entitled, "ElectrochemischeAbscheidung von Metallseleniden", by H. Von Gobrecht, H. D. Liess and A.Tausend, in Ber. Deutsche Bunsengesellschaft, Vol. 67 (1963), page 930.This article does not describe the production of photo-voltaic powergenerating cells. In accordance with this prior art technique,deposition of the less noble component and the more noble component mustbe very carefully controlled due to the difference in standardprecipitation potentials. The more noble component had to be added incarefully controlled small doses in order to operate with thistechnique.

Therefore, there has been, and is, a well recognized, but unfulfilledneed for photo-voltaic power generating means having relatively highpower generating capability per dollar of cost to produce and having aform suitable for commercial and consumer use, and for a method forproducing such means.

OBJECTS OF THE INVENTION

It is, therefore, the primary object of the present invention to providea photo-voltaic power generating means in the form of a power generatingcell which is constructed of semiconductor material having an N-typeregion and a P-type region.

It is another object of the present invention to provide a photo-voltaicpower generating means of the type stated which operates with arelatively high degree of efficiency and which can also be made at arelatively low cost, compared to conventional and proposed photo-voltaicpower generating means.

It is a further object of the present invention to provide aphoto-voltaic power generating means which can be produced in the formof a relatively flat sheet for disposition upon a surface which islocated to receive solar radiation.

It is an additional object of the present invention to provide alow-temperature method of producing photo-voltaic power generating meansand which eliminates the high temperature operation which was heretoforeemployed to produce such power generating means having semiconductormaterials.

It is also an object of the present invention to provide a method ofcathodically depositing semiconductor forming material at the cathode ofan electrolytic cell.

It is yet another object of the present invention to provide aphoto-voltaic power generating means which is created by cathodicallydepositing semiconductor forming material at the cathode of anelectrolytic cell to produce a semiconductor compound which isphotoreactive.

It is another salient object of the present invention to provide amethod of producing photo-voltaic power generating means of the typestated which is highly efficient and substantially eliminates materialwaste.

With the above and other objects in view, our invention resides in thenovel features of form, construction, arrangement and combination ofparts presently described and pointed out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described in the invention in general terms, reference willnow be made to the accompanying drawings in which:

FIG. 1 is a somewhat simplified perspective view of a photo-voltaicpower generating means in accordance with the present invention;

FIG. 2 is a somewhat schematic side-elevational view of a power cell inthe power generating means of FIG. 1;

FIG. 3 is a side-elevational view, somewhat similar to FIG. 2, andshowing a slightly modified form of photo-voltaic power cell;

FIG. 4 is a perspective view, partially broken away and shown insection, and showing a preferred power cell construction in accordancewith the present invention;

FIG. 5 is a schematic electrical circuit view showing an equivalentelectrical network for a solar energy operated cell in accordance withthe present invention;

FIG. 6 is a schematic side-elevational view showing one method forforming a photo-voltaic power cell in accordance with the presentinvention;

FIG. 7 is a schematic side-elevational view, somewhat similar to FIG. 6,and showing another modified form of creating a photo-voltaic power cellin accordance with the present invention;

FIG. 8 is a schematic side-elevational view, somewhat similar to FIG. 6,and showing another method for forming a photo-voltaic power cell inaccordance with the present invention;

FIG. 9 is a somewhat schematic side-elevational view, somewhat similarto FIG. 6, and showing still another modified form of method forcreating a photo-voltaic power cell in accordance with the presentinvention;

FIG. 10 is a schematic side-elevational view, somewhat similar to FIG.9, and showing yet another modified form of method for creating aphoto-voltaic power cell utilizing a plurality of anodes in accordancewith the present invention;

FIG. 11 is a schematic side-elevational view, somewhat similar to FIG.10, and showing another modified method of the present invention whichalso utilizes a pair of anodes;

FIG. 12 is a schematic side-elevational view, somewhat similar to FIG.11, and showing an additional modified form of the present invention forcreating a photo-voltaic power cell in accordance with the presentinvention;

FIG. 13 is a schematic diagrammatic view showing the steps utilized inthe method of the present invention; and

FIG. 14 is a chart relating the plating voltage to the current inmilliamperes per square centimeter for cadmium telluri to show how thecomposition of the cadmium telluride changes with the plating voltage.

DETAILED DESCRIPTION

Referring now in more detail and by reference characters to thedrawings, 20 designates a photo-voltaic power generating means, asdepicted in FIG. 1, in a form suitable for commercial and consumer useand configured as a sheet or panel 22. This panel 22 is sized to bedisposed upon a surface 24 as shown as the roof of a dwelling. In thedepicted application, the photovoltaic power source 20 generates poweras a consequence of having solar radiation incident thereupon. Theinvention may, of course, be utilized in a wide range of otherapplications, including heavy stationary installations, vehicles, andlaboratory uses, with other light sources and in other configurations.

The power generating means 20 comprises as a major integral componentthereof, a photo-voltaic power generating cell 26, (FIG. 2) which isformed of semiconductor material. In this respect, the sheet or panel 22may be comprised of a plurality of series-connected cells, such as thecells 26. The cell 26 in its simplest form includes an N-type region 28and a P-type region 30, which are separated by a junction 32, in amanner to be hereinafter described in more detail. While the presentinvention is effective with a hetero-junction, it is also possible toproduce the N-type region 28 and the P-type region 30 with homo-junctiontherebetween.

The term "photovoltaic" as used herein refers to a compoundsemiconductor which is capable of generating electrical power when thecompound semiconductor is subjected to the incidence of solar radiationor similar forms of light radiation. The semiconductor in its simplestform is often referred to as a "cell". In many cases the term "cell" isalso used to encompass not only the compound semiconductor, but thesubstrate and terminals or electrodes as well. In each case the cellwill have two regions, e.g. an N-type region and a P-type region,establishing a junction therebetween.

The N-type region 28 is formed of an N-type material which may compriseany of a number of well-known compositions which exhibit N-typesemiconductor properties. The P-type region 30 is formed of a P-typematerial formed of any well-known composition which exhibits P-typeproperties. In a preferred aspect of the invention, the cation ispreferably cadmium or zinc and the anion is sulfur, selenium ortellurium.

FIG. 3 illustrates a modified form of photo-voltaic power generatingcell 34 which comprises a substrate 36 formed of a relatively inertelectrically non-conductive material which is preferably transparent tolight radiation. The substrate 36 may be formed of a relatively low-costmaterial, such as any of a number of plastics, and particularly thatplastic sold under the trademark "Mylar" and the material sold under thetrade name "Kapton" of relatively low heat resistance and of low cost.However, any of a number of other substrate materials may also be usedin accordance with the present invention, and include any fiberreinforced plastic substrates, such as for example, epoxy resinimpregnated fiberglass substrates, or the like. In essence, thesubstrate should be one which is relatively inert with respect toelectrical conductivity and may be without substantial heat resistance.

Bonded to one flat surface of the substrate 36 is an electricallyconductive metal electrode 38 which may be composed of a relativelyinert electrically conductive metal, such as stainless steel, nickel orthe like. In this case, it can be observed that the electrode 38comprises a thin layer or sheet, although the electrode 38 may takeother forms and may have other positions in accordance with the presentinvention. The electrode 38 may also be secured to the substrate 36 byany of a variety of known techniques, such as metal vapor deposition,electrolytic deposition, or the like. Otherwise, the electrode 38 may beprefabricated as a strip and bonded to the substrate 36 by means ofconventional adhesives, etc.

Secured to the exposed flat surface of the electrode 38 is aphoto-electric power cell 40 which is substantially similar inconstruction to the power cell 26. In this case, the photo-voltaic cell40 is comprised of a compound semiconductor material. This cell 40 isprovided with an N-type section region 44, similar to the N-type section28, and a P-type section 46, similar to the P-type section 30, with ajunction 48 therebetween. An electrically conductive cover sheet 50 issecured to the outer surface of the section 30.

The N-type region 44 and the P-type region 46 are similarly formed inthe same manner as the N-type region 28 and the P-type region 30 wereformed in the cell 26. Moreover, in this case, the N-type region 44 andthe P-type region 46 may also have a homo-junction 48 therebetween, asin the case where the N-type and the P-type regions are formed ofsubstantially the same material. Otherwise, these two regions may beformed of different materials and have a heterojunction 48 therebetween.Finally, an electrically conductive connector 52 is connected to themetal electrode 38 and an electrically conductive connector 54 isconnected to the sheet 50. This cell 34 operates in substantially thesame manner as the cell 26 and will generate a current flow through aload connected across the connectors or terminals 52 and 54 when solarradiation is incident upon the cell 34.

The cell 26 or the cell 34 may be completely enveloped within andcontained by a container, or similar form of container means, orso-called envelope (not shown), which is formed of a materialtransparent to light radiation. The container means may be formed of anyof a number of known materials capable of passing solar radiation andincluding all forms of light radiation as for example, plastic materialincluding polyethylene sheets. Polybutyral sheets and other forms ofplastics, as well as other electrically non-conductive like transparentmaterials, may also be used in the formation of the container.

The cell 40, as well as the previously described cells may be formed ofcadmium sulfide or cadmium selenide, and preferably of cadmiumtelluride. In the depicted embodiment, the cell 40 is in the form of athin layer, although in other applications the cell 40 and the N-typeand P-type regions 44 and 46 may be configured in any appropriatemanner. As described in detail below, the thickness of the cell 40 isreadily controllable through the method of fabrication of thephoto-voltaic power generating means in accordance with the invention,thereby affording substantial economy. In any event, the cell 40, aswell as the other layers of the cell 40, as described below, may be ofthe order of 1-20 microns in thickness, although the cells willpreferably range in thickness between about 0.1 to 40 microns.

The top surface of the container means for the cells 26 or 34 and coversheet 50 would be transmissive to light and comprises an element of alight path 56 as schematically illustrated in FIG. 3 of the drawings. Anadditional or alternative light path (not shown) may be provided throughthe lower surface of the cover in which the substrate 36 and electrode38 may comprise grid structures to permit access of light and will bevery thin to minimize internal electrical resistance.

In accordance with the above, it may be observed that the photo-voltaicpower generating means of the present invention may be disposed incontact with light receiving surfaces, such as roofs of structures, inthe form of continuous panels which may be fitted to the size. Thesefeatures, among other previously described and hereinafter described inmore detail, constitute a substantial advantage of the invention interms of ease of use and economy.

As indicated previously, the cover means over the photo-voltaic cellwould be transmissive of light and comprises an element of the path 56giving access to the cell 40, that is through the P-type semiconductorregion 46. In this case, the P-type semiconductor region 46 is theregion which is exposed to light radiation, although it should beobserved that the N-type region 44 could also be the outermost regionexposed to light radiation through the path 56.

The leads 52 and 54 connect the cell 40, and hence the power generatingmeans, to an external load (not shown). This load may comprise, forexample the main power source of a vehicle or of electrical systemswithin a vehicle. In operation, light traversing the light path 56strikes the photo-cell 40 and causes a movement of electrons from thesemiconductor material of the P-type region 46 across the junction 48 tothe N-type region 44, under well-known phenomena of photointeractionwith semiconductive materials. Consequently, a migration of electrons tothe minus terminal 52 occurs and a current appears in the leads and inthe load.

In the embodiment of the cell illustrated in FIG. 3, where the lightpath is designated 56, at least the electrode 50 must be transparent. Inthis case, the electrode 38 and the substrate 36 would not have to betransparent. However, in accordance with the present invention, the cellcould have a transparent electrode 38 and a transparent substrate 36,formed of a conductive glass or transparent plastic substrate, asdescribed in more detail hereinafter. In this construction, the cellwould respond to a light path designated as 56' in FIG. 3 of thedrawings. However, it should be recognized that all components of thecell 34 could be transparent.

Where the cell 34 is constructed so that it responds to the light path56, the narrow band gap material will be incidental to the electrodes 38and the wide band gap material will be incidental to the electrode 50.When the cell 34 is constructed so that it responds to the light path56', the wide band gap material will be incidental to the electrode 38and the narrow band gap material will be incidental the electrode 50. Inessence, the wide band gap material will always face the source oflight. In the present invention, a CdS layer has a wider band gap thanthe CdSe layer, which in turn, has a wider band gap than a CdTe layer.

In accordance with the invention, the electrode 50 need not be formed ofa metal, but could be formed of a conductive transparent oxide ashereinafter described. Again, the electrode 38 could be formed of aconductive transparent oxide. In addition the substrate 36 could beconducting and constitute an electrode, thereby eliminating thenecessity of the electrode 38.

One of the preferred embodiments of a photo-voltaic power cellconstruction in accordance with the present invention, is more fullyillustrated in FIG. 4 of the drawings. In this case, the preferredembodiment of the power cell is designated by reference numeral 57 andincludes a substrate 58 which is electrically non-conductive, such as aglass substrate. This substrate 58 is preferably relatively thick withabout a thickness of one-eighth inch. A metallic electrode 60 isdisposed on one flat surface of the substrate 58 and this electrode 60,in the form of a grid, is comprised of a plurality of parallel spacedapart transversely extending strips 62 and a plurality of paralleltransversely spaced apart longitudinal extending strips 64. In thepreferred aspect of the invention, the strips 62 and the strips 64 arelocated in an essentially perpendicular relationship.

A conductive coating 66 is applied to the surface of the substrate 58 inwhich the metal grid 60 is applied and this substrate 58 is preferablyan electrically conductive coating comprised of stannous oxide dopedwith antimony, or indium oxide doped with tin. The electricallyconductive coating essentially completely covers the entire innersurface of the substrate 58 except for the portions of the grid incontact therewith and completely covers the metal grid 60 and is inelectrical contact therewith while the grid 60 is on the inner surfaceof the substrate 58.

This last substrate 58 may be formed of any transparent substratematerial, as for example, polymethylmethacrylate, or the like. Inaddition, the substrate 58 could actually form part of a basic cellbuilding block in the form of a glass roof or wall tile. In any event,it is important that the substrate is sufficiently transparent to admitthe passage of light, when the cell is oriented for passage of lightthrough the substrate.

The coating 66 which also faces the source of solar energy is coatedupon the substrate 58, preferably by vapor coating, in the form of auniform thin film of electrically conductive material which ispreferably antimony-doped tin oxide, or indium oxide doped with tin. Inaccordance with the present invention, it has been found that it is notnecessary to use a metal as an electrode and that a relatively thicktransparent substrate can serve as the electrode when made electricallyconductive through application of a conductive oxide. The conductiveoxide is an N-type material and therefore the conductive oxide must bein contact with the N-region or otherwise another junction would beestablished.

The grid 60, often referred to as a "bridge", is on the surface of thesubstrate 58 and located between the substrate 58 and the tin oxidecoating 66 in order to lower the ohmic resistance. In this way, the grid60 becomes a first electrode which has a resistivity well below one ohmper square inch. A terminal 68 extending from one portion of the grid 60serves as a first terminal for making electrical connection to the cell.A flat bus bar (not shown) may also extend around the periphery of theterminal portions of the grid 60 to serve as one of the two terminalsfor providing the electrical connection to the cell.

The photo-voltaic cell 57 will also include the cell structure of thetype illustrated in FIG. 2 of the drawings. In this case, the cellstructure includes an N-type section 70, equivalent to the N-typesection 44 in FIG. 3, and a P-type section 72, equivalent to the P-typesection 46 in FIG. 3 of the drawings, with a junction 74 therebetween.The N-type section 70 may be comprised of, e.g, cadmium selenide orcadmium telluride, whereas the P-type section may be formed of the samematerial with a homo-junction or a different material with aheterojunction. A relatively thin metallic film 76, is applied to theouter surface of the P-type section 72, in the manner as illustrated inFIG. 4. This outer metallic film 76 constitutes the rear electrodeassembly and is provided with an electrically conductive lead wire 78.

It can be observed that the construction of the cell of FIG. 4 enableslight to pass through the substrate 58, thereby eliminating the need ofa third metal substrate. Moreover, it can be observed that the grid 60is also electrically conductive and in conductive relationship to theN-type region 70 through the conductive transparent oxide film 66. Theouter metallic film 76 may have a reflective surface facing thesubstrate 50 so as to cause reflection of the light which entered thecell and thereby cause greater energy conversion efficiency.

A relatively high efficiency value for a polycrystalline photo-voltaiccell can be achieved by a combination of factor including the use of athin layer of cadmium telluride facilitating maximum transport ofphotons to the junction region. The use of a thin film of modifiedmetal, as for example, vapor deposited nickel, performs as ananti-reflection coating on the surface of the glass facing the sun rays.The disclosed structure is quite effective in that it reduces ohmiclosses in the two electrodes and the semiconductor material.

This form of cell structure is highly advantageous over previous priorart cell structures of the single crystal type in that the cellstructures described herein include substantial economies which becomepossible through the deposition of thin layers of one or more costlyactive materials on an inexpensive glass or transparent plasticsubstrate. In accordance with this latter embodiment of the invention,this embodiment provides the ability to make a large integrated areacell without the necessary recourse to interconnecting a multiplicity ofsmall or independent units in a connected arrangement. Moreover, thiscell structure includes the possibility of the employment of printedcircuit type conductors to connect a plurality of individual cells on atile or similar substrate in series or series-parallel arrangement.

FIG. 5 illustrates, in schematic form, a preferred electricalconfiguration of at least one or more cells connected in accordance withthe present invention. It has been well established that absorption ofphotons having wave lengths shorter than the optical band-gap createselectron-hole pairs in a crystal lattice of the semiconductor material.A built-in field provided by the P-N junction, e.g. the P-N junction 32or the junction 48, or otherwise, a Schottky barrier, separates theelectrons and the holes, generating a photovoltage which biases thejunction in a forward direction. Thus, in this way, a solar cell of thetype proposed by the present invention can be represented by theequivalent circuit in FIG. 5 of the drawings.

More fully considering FIG. 5, it can be observed that each of the cellsare designated by reference numeral 80 which functions as a currentgenerator per unit area. These cells have a diode 82 connected inparallel therewith in the manner illustrated in FIG. 5 of the drawings.The diodes 82 are of unit area with respect to the current generators,such as the photovoltaic cells 80. In this respect, it can be observedthat while the cells 80 are connected across the diodes 82 in parallelrelationship, the opposed terminals of the diode 82 are connected to apositive line 84 and a ground line 86. Resistors 88 and 90 represent thesheet resistance of the electrodes and of the adjacent electricallyneutral portions of the semiconductors bordering the built-in fieldregion. Resistors 92 and 94 are representative of the contactresistances per unit area of the neutral regions with these electrodesand the resistance per unit area of these neutral regions. Each cell hassimilar resistive functions and diode functions in the manner asillustrated in FIG. 5 of the drawings.

For optimal conversion efficiency, the resistances 88 and 90, as well asresistances 92 and 94, the latter of which constitute parasiticresistances, should be made as small as possible. The selection of thesemiconductor material for optimizing similar energy conversion thusinvolves maximizing the effective full type conversion intoelectron-pairs for solar radiation. In other words, this efficiency iscreated by maximizing the current generator, e.g. the solar cells 80 andmaximizing the forward resistance of the various diodes 82. Themaximization is required with respect to the solar cells 80 and thediode 82 and these requirements are interrelated, resulting in acompromise on the bandgap of the semiconductor material which is chosenwith decreasing band-gap as more radiation is absorbed. However, theinternal resistance of the barrier decreases, leading to optimalband-gaps for cadmium telluride of approximately 1.5 eV for solarradiation conversion at the earth's surface, on a cloudy day.

The technique for making the photo-voltaic power cells in accordancewith the present invention is more fully illustrated in FIGS. 6-12 ofthe drawings with a schematic flow diagram thereof illustrated in FIG.13 of the drawings. In essence, the present invention provides for thecontrollable electrochemical production of junctions of cadmium andzinc-type compound semiconductors used as photo-voltaic cells. Inaccordance with the invention, a semiconductor compound material isformed at the cathode where both the more noble components and the lessnoble components are discharged.

Referring now to FIG. 6, 100 designates a container, such as a beaker,formed of a relatively inert material. Located within the container 100is a cathode 102 which is similarly formed of a relatively inertmaterial, nickel as shown. However, any other form of metal electrodewhich is inert to the reaction, such as steel or glass or plasticprovided with a conductive oxide coating, for example, may be used. Alsolocated within the container 100 is an anode 104 which may also beinert, or otherwise the anode may be formed of cadmium or zinc orselenium or tellurium as hereinafter described. As illustrated, theanode 104 is formed of an inert platinum material. Both of theelectrodes 102 and 104 are disposed within an electrolyte 106, ashereinafter described, and both the anode and the cathode areelectrically connected through a source of electrical current 108.

This particular arrangement of FIG. 6 represents a simplified systemwhich illustrates the formation of a coating at the cathode 102. By wayof example, it is possible to electrochemically deposit sulfur on thenickel cathode 102 to form a sulfur coating, designated as S in FIG. 6,through the use of an electrolyte such as SO₂ in H₂ O. In this way, thereaction which proceeds is represented by:

    4e.sup.- +4H.sup.+ +H.sub.2 SO.sub.3 =3H.sub.2 O+S.

This reaction demonstrates that sulfur is reduced during deposition atthe cathode. Similarly, the H₂ SO₃ is oxidized to H₂ SO₄ at the anode.In this case, deposition would occur preferably at about 10° C. to about20° C., with about three to six volts applied across the anode andcathode, along with a current density of 0.1 amp. per square centimeter.An optimal deposition of the sulfur occurs from a 1 mg 1⁻¹ solution ofSO₂ in water.

In the event that it was desired to form cadmium sulfide, as opposed toa mere sulfur layer at the cathode, the electrolyte could be changedfrom SO₂ in water to SO₂ +3CdSO₄.4H₂ O. In the ionic dissociation of thecadmium sulfate in water, positively charged cadmium ions are formed.These cadmium ions are attracted to and discharged at the cathode onwhich sulfur is also being deposited simultaneously. Thus, the cadmiumand the sulfur will combine as they are simultaneously discharged at thecathode to form a layer of cadmium sulfide on the cathode. In this way,it is possible to form a film of cadmium sulfide with any desiredstoichiometry, as established through the concentrations of the solutesused in the electrolyte.

FIG. 7 illustrates a system similar to FIG. 6, except that in this casethe cathode which is employed will constitute the metal upon which acoating is desired to be formed. It can be observed that the cathode 102is formed of cadmium and with the aforementioned reaction, sulfur can becathodically deposited as along with cadmium a film upon the cadmiumcathode to obtain, for example, cadmium sulfide.

FIG. 8 illustrates another embodiment of the method of making a cadmiumsulfide compound film on an inert cathode, which in this case, is shownas glass having a conductive oxide coating thereon. Again, the anode isalso formed of an inert material, such as platinum. In order to producethe cathodic coating of cadmium sulfide, the sulfur is introduced into asolution of the electrolyte in the form of SO₂ in H₂ O, and the cadmiumis introduced in the form of 3CdSO₄.4H₂ O, dissolved in this solution aspreviously described. It should also be observed that cadmium tellurideand cadmium selenide, etc., zinc sulfide, zinc selenide and zinctelluride could be formed in the same way. Thus, in order to form acadmium telluride coating on the cathode 102, the electrolyte wouldconstitute tellurous acid as the source of tellurium and cadmium sulfateas the source of cadmium. In this way, the positively charged cadmiumions which are thus formed would be discharged at the cathode. In likemanner, the tellurium would be deposited at the cathode andsimultaneously react with the cadmium to form the cadmium telluridefilm.

It can be observed that it would be necessary to plate out the cadmiumand the tellurium, or the other components used, in the desiredstoichiometric amounts. However, the standard voltage required forplating out the cadmium and tellurium would be different. For example, amore negative voltage would be needed for the less noble component, asfor example, cadmium, than for the more noble component, as for example,tellurium or selenium. While there is somewhat of a compensating effectwith respect to the deposition voltages when a semiconductor componentis formed, it is usually desirable to decrease the concentration of themore noble component. Thus, in the case of producing cadmium telluride,the amount of tellurium in solution would be decreased with respect tothe amount of cadmium.

In order to form a cadmium telluride or similar photo voltaic device, asillustrated in FIGS. 2, 3 or 4, a first layer of cadmium telluride wouldbe plated on the nickel anode 104, in the manner as previouslydescribed. The film thus formed on the cathode would be produced as anN-region or a P-region, depending upon the ratio of the cadmium andtellurium. After forming the first cadmium telluride layer on thecathode, as for example the glass with oxide coating cathode in FIG. 8,a second electrolyte similarly including the same compositions toproduce the source of cadmium ions and the tellurium ions would also beused. However, the concentration ratio of the cadmium and tellurium inthe second solution would be different from that of the first solutionin order to form the other of the P-type region or N-type region. Thus,for example, if a first film of cadmium telluride were plated onto thenickel cathode 102 with, e.g., 50.01% cadmium, this film wouldconstitute the N-type layer 28. When the second film of cadmiumtelluride from the second electrolyte is placed on the first film, thissecond film could have a lower concentration of cadmium, as, forexample, 49.99%. In this case, the second film would function as, andconstitute, the P-type layer 30. Thus, it can be observed that by merelycontrolling the stoichiometry of the metal component, e.g. cadmium, andthe non-metal component, e.g. tellurium, or otherwise the ions of anyother metal and non-metal components used in accordance with the presentinvention, it is possible to produce either an N-type layer or a P-typelayer In accordance with this example, it can be observed that the twofilms thus formed on the nickel cathode 102 will form a homojunction 32therebetween.

It should be observed in accordance with the present invention that itis possible to produce the N-type region and the P-type region fromdifferent materials with a heterojunction therebetween. In this case,e.g., cadmium selenide would be formed as a first film on the cathode102, which is glass with a conductive oxide coating as shown.Thereafter, the electrolyte would be changed to plate out, e.g. cadmiumtelluride. The cadmium telluride would then be plated onto the firstlayer of cadmium selenide. In this way, the concentrations of thecadmium with respect to the tellurium and the selenium would bestoichiometric adjusted so as to create both an N-type region and aP-type region Thus, the cadmium selenide layer could operate as either aP-type region or an N-type region, but more preferably an N-type region,and the same holds true of the cadmium telluride layer, which wouldpreferably be a P-type layer.

FIG. 9 illustrates another alternatively technique for producing acathodically formed film in accordance with the present invention. Inthis case, the cathode is also inert, as, for example, the glass withthe conductive oxide coating as shown. The anode, in this case, would beformed of the metal component, as, for example, a solid cadmium or zincsheet, or otherwise a cadmium or zinc-plated sheet. The electrolytewould be comprised of those materials which provided the non-metalcomponent of the compound. Thus, in the case of sulfur, the electrolytewould comprise a solution of SO₂ in water. In this way, cadmium sulfidewould be formed at the cathode. Again, tellurous acid could be used asthe electrolyte and, in which case, cadmium telluride would be formed onthe nickel cathode.

With cadmium sulfide, it has been found that the cadmium sulfide can beformed on the cathode with a layer of a thickness of about 5 microns forpreferred results. These layers are obtained from a 5% solution of SO₂at about 45° C.

With the embodiment of FIG. 9, as well as some of the other embodimentsherein, it is also possible to utilize cadmium and similar metal anodescontaining dopants. Thus, indium as a donor dopant could be combinedwith the cadmium as a cadmium indium alloy to be used as the anode. Inthis way, the electrochemical process of the invention has the advantageof forming a cadmium sulfide film which contains indium in solidsolution. By choosing the proper concentration of the cadmium-indiumalloy, the indium concentration in the cadmium sulfide, or otherwisecadmium telluride, etc., can be regulated.

Thus, it can be observed that those systems illustrated in FIGS. 6, 7, 8and 9 are all effective in forming the desired photo-voltaic filmmaterial on the cathode. Moreover, in each of these systems, by changingthe electrolyte it is possible to form a second film in the same manneras previously described. Thus, if the two films are formed of the samematerial with one being of the N-type and the other being of the P-type,they will form a homojunction therebetween, and with different materialsthey will form a heterojunction therebetween.

As also previously described, the N-type region and the P-type regioncan be formed merely by adjusting the stoichiometry of the componentsused. However, it is also possible to use any of several dopants in thetwo regions. Thus, one of the regions could be doped with indium,aluminum or gallium, etc., as donors, or with phosphorus, arsenic orantimony, etc., as acceptors.

The present invention is primarily effective for use in producingcathodically formed films with cadmium and zinc ions and sulfur,tellurium and selenium ions. In addition, mixed crystals of the typesCd(S,Se), Cd(S,Te) Cd(Se,Te), Cd,Zn(Te), Cd,Hg(Te) and Cd,Mg(Te), etc.,can be produced. Thus, any combination of mixed crystals formed of ionsof cadmium, mercury, magnesium, zinc and any form of mixed crystals, as,for example, those formed of ions of sulfur, selenium and tellurium maybe produced by the present invention. These substances may be pure ordoped with those donors or acceptors as previously described or anyother form of effective and acceptable donor or acceptor.

As indicated above, electrolytic deposition on a conducting cathodepermits ions from both the metal and non-metal components in theelectrolyte to be simultaneously discharged at the cathode and formed asa semiconductor compound material on the cathode. As also indicted, SO₂may be used as the electrolyte in order to form a sulfide layer, aspreviously described. Cadmium sulfate is also used in combination withthe SO₂ in order to form the cadmium sulfide layer. When forming thevarious cadmium salt films as semiconductor compounds, various acids,such as H₂ SeO₃, H₂ SO₃ or H₂ TeO₃ may be used, or otherwise thealkaline salts of these acids may be used with an inert anode. Inaddition, solutions in acid of SO₂, SeO₂ or TeO₂ may be utilized with aninert anode. The composition of the deposited film is controlled throughthe composition of the electrolyte as described. Alternatively, it ispossible to use as an electrolyte a solution of SO₂, SeO₂ or TeO₂ inwater with an anode of Cd(Cd,Zn) (Cd,Hg) or (Cd,Mg), etc.

The ions formed by the metal components, e.g. cadmium and zinc, and theions formed by the non-metal components, e.g. sulfur, selenium andtellurium, in solution cannot necessarily be characterized as singlecations and anions. Generally, the cadmium and zinc in solution willform single cations since they are generally positively charged, e.g.,Cd⁺⁺ and Zn⁺⁺. In many cases the non-metal components provide ions,e.g., S⁼ and Se⁼. Tellurium, for example, can adopt several valencestates as Te⁼. However, TeO₃ ⁼ complex ions can be formed. Moreover,Te⁺⁴ ions could be formed with TeO₂ in hydrofluoric acid. In this case,TeF₄ would form which dissociates to produce Te⁺⁴ in solution.

The metal and non-metal components constitute the substantial amounts ofthe components in the cell thus formed. As indicated above, the metaland non-metal compounds can alloy or form mixed crystals with theelements and thus the mixed crystals or alloys constitute thesubstantial amounts of the components in the cell. As also indicated,various dopants may be introduced into the layers which form thesemiconductor cell. While these dopants may actually alloy or form mixedcrystals with the metal and non-metal components, they are notconsidered to form part of the mixed crystals or alloys in substantialamounts.

The electrochemical principles which might be applicable to explain theplating of both the metal and non-metal components as a semiconductorcompound on the cathode are not fully understood. Nevertheless, it hasbeen established that these components do plate out at the cathode toform a semiconductor compound. With respect to the ions of the metalcomponents, these ions would normally be attracted to and discharged atthe cathode. The reasons for the discharge of the ions of the non-metalcomponents is more complex.

The non-metal components present ions in solution in the presence ofhydrogen. Thus, for example, the non-metal components are introduced inan acid form, in most cases presenting an available source of hydrogen.It has been theorized that the hydrogen in proximity to the cathode aidsin the reduction of the non-metal ions in proximity to the cathode.Thus, for example, TeO₃ ⁼ +6H⁺ provides Te⁺⁴ ions which are available atand become discharged at the cathode. Nevertheless, while the exactprinciples may not be fully understood, it has been established that thecathodic formation of the semiconductor compound material does occur.

Various examples will now be given to show how different photo-voltaiccells in accordance with the present invention may be manufactured.

EXAMPLE 1

A liquid electrolytic bath having an anode and cathode was set up asillustrated generally in FIGS. 6-9. The cathode consisted of a sheet ofglass having deposited thereon a coating of electrically conductive,light transparent indium tin oxide (sometimes called ITO). Such ITOcoating is indium oxide doped with tin and has the empirical formula:In₂ O₃ :Sn. It had a thickness of approximately 4,000 A (Angstromunits), 1 Angstrom unit being 10⁻⁸ cm. The anode consisted of a bar oftellurium having the dimensions of approximately 11/4"×1"×1/2". Thedistance between the electrodes was about 2" to 3". In order to stir thebath, a magnetic stirring rod or bar was used. The electrolytic bathconsisted of 250 ml of 1.0 N cadmium sulfate. An N (normal) solution wasobtained by dividing the molecular weight by the valence of the compoundto yield the number of grams required to give the specified normality.To this weight of material, water was added until 1 liter of liquid wasobtained. To such cadmium sulfate solution was added 30 ml of 0.01 Nindium sulfate.

In order to purify the cadmium sulfate-indium sulfate solution prior tousing it to make a cell, first a piece of cadmium metal, 1"×2"×1/16" wasleft in the solution for 1/2 day to about 2 days. Then a preliminarydeposition process was run under the same conditions as described in thenormal cell manufacturing process described below except that thevoltage was -0.680 volts vs. SCE measured between the cathode and astandard reference calomel electrode (sometimes called SCE), and was runfor two to three hours.

The adjustment of the pH to the desired pH was accomplished by addingconcentrated sulphuric acid dropwise to the solution and the pH wasmeasured continuously with a pH meter until the desired pH level wasreached.

The first step was to plate a layer of n-type cadmium telluride having athickness of approximately 2,000 A by plating for 60 minutes at a pH of2.85 and at a temperature of 90° C. (Celsius) onto the ITO layer servingas the cathode. The plating voltage for such plating step was -0.635volts.

In the second step, the plating voltage was turned off and the cadmiumtelluride plated cathode was left in the bath for approximately 5seconds. During such step the surface of the n-type cadmium telluridelayer appeared to have been converted to a p-type tellurium or cadmiumtelluride layer.

In the third step, the previously prepared cadmium telluride p-njunction was immersed in a bath of 100 ml of 1 N sodium sulfate to which10 ml of 0.01 N arsenic pentoxide had been added. The pH of the bath was2.4, and the cathode coated with cadmium telluride was immersed in suchbath for approximately 20 seconds. Both before and after the immersionin the sodium sulfate-arsenic pentoxide solution, the cell was rinsedfor about half an hour in deionized water.

Finally, during a fourth step the cadmium telluride plated cathode waselectroplated with tellurium. The tellurium was provided by anadditional tellurium anode as illustrated, for example, in FIG. 11. Suchelectroplating step was carried out in an electrolytic bath of 1 Nsodium sulfate at a pH of about 2.45 and at a temperature of 90° C. toproduce a layer of tellurium of approximately 3,500 A thickness. Suchplating was carried out for about 5 minutes at a voltage of -0.550volts, and then for 30 minutes at a voltage of -0.620 volts. Finally, aback electrode was attached to the cell in the form of Aquadag whichprovides an ohmic contact with the tellurium. Such Aquadag is awell-known commercial product which consists of a suspension ofcolloidal graphite in water.

The photo-voltaic cell constructed in accordance with the foregoingsteps when illuminated by a simulated solar radiation lamp, i.e., aGeneral Electric RS/HUV Mercury Lamp simulating AM1, exhibited an opencircuit voltage typically of 350 millivolts and a short circuit currentof 130 to 150 microamperes per square millimeter on the average. Lightentered the cell through the ITO layer.

The specific details set forth in the foregoing example can be variedsubstantially without materially decreasing the performance of the cellmade by such method. For example, the thickness of the indium tin oxidelayer may be in the range of about 3,500 to 5,000 A. Thinner layers givehigher sheet resistance and less light absorption; thicker layers havethe reverse properties. The concentration of the cadmium sulfate may bebetween about 0.25 N and 3.0 N and the concentration of the indiumsulfate may be in the range of about 0.005 N to 0.02 N. It should benoted that the indium sulfate concentration was determined on theassumption that the plating efficiency was about 50%; and, therefore,the initial electrolyte concentration is about twice that which isconsidered achieved. The desired concentration of the indium donors inthe cell is between 10¹⁴ and 10¹⁸ molecules per cubic centimeter with apreferred concentration of about 10¹⁷ molecules per cubic centimeter.The pH range for the cadmium sulfate solution may be between about 2.50and 3.50. The present data indicates that increasing the pH will improvethe photo-voltaic cell characteristics but it reduces the speed at whichdeposition takes place. The temperature range of the cadmium sulfatesolution during deposition may be between about 20° C. and 100° C. Thetime of plating of the n-type cadmium telluride layer may be betweenabout 20 and 120 minutes. The plating voltage for the n-type cadmiumtelluride layer may be between about -0.500 and -0.645 volts. The rangeof the thickness of the n-type cadmium telluride layer may be betweenabout 1,000 and 4,000 A. The current density may be up to about 0.7milliamperes per square centimeter of plated area.

With respect to the formation of the p-type cadmium telluride ortellurium by immersion in the solution, the range of time which thecathode may be left in the initial electrolytic bath may be betweenabout 0 seconds to about 2 minutes. By zero seconds it is meant that thecathode is removed rapidly from the solution before the voltage isturned off. It is believed that the deposited n-type cadmium layerduring the immersion period is changed to a p-type layer because some ofthe tellurium replaces the cadmium and indium which go into solution.With respect to the immersion of the coated cathode in the sodiumsulfate-arsenic pentoxide solution, the sodium sulfate may have aconcentration in the range of between about 0.25 N and 3.0 N. Similarly,the concentration of the arsenic pentoxide may be between about 0.005 Nand 0.02 N. Similar to the indium concentration, the plating efficiencyof the arsenic is assumed to be approximately 50% and therefore, theresulting concentration is twice what would normally be required. Theimmersion time in the sodium sulfate solution may range from about 1/2second to about 40 seconds. The pH range of the sodium sulfate solutionmay range from about 1.9 to 2.4.

With respect to the deposition of the tellurium during the fourth step,the voltage may range from about -0.550 to -0.620 volts and the pH mayrange from about 2.6 to 3.5. The resulting thickness of the telluriumlayer may range from about 500 A to 9,000 A. The deposition time mayrange from about 1 minute to 60 minutes.

EXAMPLE 2

The same procedure as set forth in Example 1 was followed except thattellurium dioxide in powdered form was added in sufficient quantities tothe electrolytic bath so that there was obtained a saturated solution ofthe tellurium oxide. In other words, there was solid tellurium dioxidein equilibrium with the solution. The performance of the cells producedunder such conditions did not vary substantially from the cells producedas set forth in Example 1.

EXAMPLE 3

The same procedure as set forth in Example 2 was followed except thatthe tellurium electrode was replaced with an inert carbon anode so thatthe tellurium came solely from a saturated solution of telluriumdioxide. A cell produced with such method did not vary substantiallyfrom the cell produced as set forth in Example 2. However, the limitedexperimentation conducted under such circumstances indicated that theelectrolytic bath conditions such as anodic processes inhibited thedissolution of additional tellurium dioxide in solution to replace thetellurium deposited at the cathode. Therefore, the bath would have to befrequently replaced.

EXAMPLE 4

The method set forth in Example 1 was followed except that it wasdesired to place p-type cadmium telluride directly on the n-type indiumtin oxide layer. Consequently, the electrolytic bath was made up of1,000 ml of 0.5 N cadmium sulfate to which was added 60 ml of 0.01 Narsenic pentoxide and the pH was adjusted to 3.0. Rather than using amagnetic stirring bar, the stirring of the bath during deposition wasaccomplished by recirculating the solution bath through a glass woolfilter with a pump. P-type cadmium telluride was deposited at -0.635volts for approximately 60 minutes. A subsequent tellurium layer wasplated at a pH of 2.2. The resulting cell had an open circuit voltage ofapproximately 410 millivolts and a short circuit current of 130 to 150microamperes per square millimeter of illuminated area.

With respect to such method set forth in the foregoing Example 4, forthe p-type CdTe deposition, the pH range may be about 2.4 to 3.5 and thevoltage range may be from about -0.555 to -0.640 volts. The ranges forthe tellurium deposition are the same as those noted above in Example 1.

EXAMPLE 5

Another photo-voltaic cell was made by the method of the presentinvention by using the first step of Example 1 to lay down an initiallayer of n-type cadmium telluride; however, the p-type layer of cadmiumtelluride was deposited by using the same bath as set forth in Example4. With respect to a cell formed under such conditions, the measuredopen circuit voltage was approximately 300 millivolts.

EXAMPLE 6

Another example of the method of the present invention was to utilizethe cathode described in Example 1 except that in place of the indiumoxide doped with tin, tin oxide doped with antimony was utilized to givea light transparent, electrically conductive n-type layer on a glasssubstrate.

EXAMPLE 7

Another example of the method of the present invention of makingphoto-voltaic solar cells was to utilize the cathode as described inExample 6 by utilizing a layer of tin oxide doped with antimony. Likethe Example 2 above, the electrolytic bath was saturated with telluriumdioxide but unlike Examples 1 or 2, an additional anode was utilizedconsisting of carbon or stainless steel having the dimension ofapproximately 11/2" long×1" wide×1/2" thick. With respect to the use ofthe carbon anode, the current was adjusted with respect to the carbonand tellurium anode so that the carbon anode current was approximatelytwice the tellurium anode current. With respect to the cadmium sulfatebath, the concentration of the cadmium sulfate was 0.50 N. Unlike any ofthe foregoing examples, no dopants were added; however, the temperaturewas the same as Example 1. Initially, an n-type layer of cadmiumtelluride was deposited by adjusting the pH to 2.9 to 3.0 and applying avoltage of -0.610 to -0.620 volts and carrying out such deposition forabout 60 minutes. Then a p-type layer of cadmium telluride was depositedby adjusting the pH to about 2.3 and applying a voltage of about -0.480to -0.500 volts and carrying out the deposition for about 30 minutes.The resulting cell exhibited an open circuit voltage of approximately100 millivolts.

As indicated above, in the foregoing example an n-p homojunction ofcadmium telluride was produced by changing the plating voltage and thepH to convert the n-type cadmium telluride to a p-type cadmium telluridebeing deposited without the use of doping.

In this connection, reference is now made to FIG. 14 illustrating acurve 140 which is a function of the deposition or plating voltage asmeasured between the cathode and an SCE, that is a standard or saturatedcalomel electrode as a function of the current in milliamperes persquare centimeter. Dotted vertical line 141 shows pure tellurium beingobtained to the left of the line, while to the right of dotted slantedline 142, pure cadmium is deposited. Between lines 141 and 142, there isdeposited cadmium telluride; thus near the dotted lines 141 there is aslight excess of tellurium in the cadmium telluride, while near line 145there is a slight excess of cadmium in the cadmium telluride.

Adjacent the curve 140 are the letters n and p to denote where n-typecadmium telluride of p-type cadmium telluride is deposited as a functionof the plating or deposition voltage (V_(dep)). The chart of FIG. 14 wasobtained with a pH of 3.4 and with an electrolyte containing cadmiumsulfate 1.2 m, that is 1.2 molar with saturated tellurium oxide (TeO₂)and a temperature of 85° C.

The curve of FIG. 14 is an experimental curve for the conditionsrecited. Also, the rest potential is the potential measured by arecorded immediately after switching off the current. Thereupon, theresulting curve plotting voltage as a function of time changes slopeabruptly at a point accurately defining the par ticular rest potential.The theory shows slanted dotted lines 145 to the abscissa relatedeposition potential to rest potential.

It will be realized that this curve will change with variations of theplating temperature, the pH and the concentrations of the reactants.Hence, the various parameters given for Example 7 are not those forwhich the chart of FIG. 14 was obtained but the results are similar.

However, it will be evident that this chart demonstrates that p-nhomojunctions of cadmium telluride may be obtain simply by changing thedeposition or plating voltage, or by changing the pH or other variablesof the plating process. Thus, for a lower pH (high acidity) the curve140 will be higher.

It will also be noted that the set of dashed lines 145 interconnectingvarious points on the curve 140 to various points on the voltage scaleare inclined with respect to a vertical line. However, they do correlatethe voltages with the various points on the curve. Thus, for the chartof FIG. 14, at voltages more negative than -0.3 volts, n-type cadmiumtelluride will be deposited. At voltages more positive than -0.3 volts,a p-type cadmium telluride will be obtained.

EXAMPLE 8

In order to produce a Schottky barrier solar cell by the method of thepresent invention, the ITO cathode described in Example 1 was directlyplated with tellurium as described in the fourth step of Example 1,except that the pH was about 2.2 and the plating was carried out at a-0.550 volts for about 20 minutes. The resulting cell when tested,exhibited an open circuit voltage about 100 millivolts.

EXAMPLE 9

Another example of the method of the present invention is to deposit onthe cathode described in Example 1, a layer of cadmium telluride withoutdoping and with the pH and plating voltage adapted to produce intrinsiccadmium telluride, i.e., cadmium telluride which is substantiallystoichiometric in proportion and having little, if any, n-type or p-typecharacteristics. Subsequent to the deposition of such intrinsic layer ofcadmium telluride, a p-type layer of cadmium telluride may be deposited,as set forth in Example 4 above, forming what is known as a P-I-N cellstructure.

EXAMPLE 10

Another example of the method of the present invention is to utilize thesame process as set forth in the foregoing example except that in placeof the p-type cadmium telluride layer which is deposited as the finalstep, a layer of tellurium may be plated instead as described in thefourth step of Example 1.

EXAMPLE 11

In making another Schottky barrier junction by the method of the presentinvention, an n-type or p-type layer is deposited by electroplating ontoa suitable metal substrate. The metal may, for example, consist ofnickel, steel, cadmium, platinum, gold, or silver. The semiconductivelayer to be deposited may, for example, consist of n-type or p-typecadmium telluride, either of which may be deposited in the mannerpreviously described. In the case of an n-type cadmium telluride layer,an n-type donor, such as indium, may be added. On the other hand, in thecase of a p-type semiconductor layer, a p-type acceptor, such asarsenic, may be added. The light preferably impinges from the surface ofthe semiconductive layer. The electrodes are applied as explainedhereinbefore. Instead of using cadmium telluride, it is also feasible toutilize cadmium sulfide (CdS), cadmium selenide (CdSe) or zinc selenide(ZnSe).

EXAMPLE 12

Instead of varying the plating voltage to deposit cadmium telluride ofn- or p-type as previously explained, it is also possible to vary theconcentration of the cadmium in the electrolyte to obtain n-type orp-type cadmium telluride layers. Thus, a first layer may be deposited ona substrate, as previously explained, from a solution having a pH of 2.5and saturated with tellurium dioxide (TeO₂). The electrolyte contains acadmium ion concentration of 0.1 M. The current density for the platingis 0.1 mA per square centimeter of plated area. This will produce ann-type cadmium telluride layer. The plating time is about one hour toone and one-half hours.

Subsequently, a second layer is deposited from a similar solution. Theonly difference is that the cadmium ion concentration is 0.001 M withthe same current density and the same plating time. This, in turn, willprovide a p-type cadmium telluride layer. Hence, a homojunction deviceis provided having an n-p junction between two layers of cadmiumtelluride. Similarly, the change in cadmium telluride stoichiometry canbe achieved by varying the pH of the electrolyte. For example, the lowerthe pH, the more tellurium dissolved in the cadmium sulfate electrolyteand more tellurium is deposited with the cadmium telluride.

The method for producing the photo-voltaic power cells of the presentinvention can also be effectively operated with a plurality of anodes,as illustrated in the arrangements of FIGS. 10 and 11. In this case, themethod would also utilize a container 110, such as a beaker, equivalentto the container 100. Moreover, in the arrangement illustrated in FIG.10, a relatively inert cathode 112, as, for example, a cathode formed ofglass with a conductive oxide coating, as shown, would also be utilized,along with a neutral anode 114 formed of an inert material, as, forexample, platinum as shown. In addition, a second anode 116 formed ofcadmium would be utilized. The two anodes 114 and 116 are connected tothe cathode 112 through a source of electrical current 118.Potentiometers 120 and 122 are respectively connected to the anodes 114and 116 and to the source 118, in the configuration as illustrated inFIG. 10. Also, the cathode 112, along with the anodes 114 and 116, arealso disposed in a suitable electrolyte 124, as those electrolytesheretofore described and as hereinafter described.

The anode 116 which is formed of cadmium may otherwise be acadmium-plated anode. In like manner, the anode 116 could be formed ofan alloy of cadmium with a desired dopant. Tellurium ions would beprovided in solution, as, for example, by a tellurous acid composition.By carefully controlling the current flow to the respective anodes 114and 116, it is possible to introduce cadmium into solution from theanode 116. In this way, the tellurium ions contained in the tellurousacid will be discharged at the cathode 112, and in like manner thecadmium entered into solution from the anode 116 will also be dischargedat the cathode 112. In order to form cadmium sulfide or cadmiumselenide, H₂ SO₃ would be used to form the cadmium sulfide and H₂ SeO₃would be used to form the cadmium selenide.

Again, a first layer could be formed on the cathode 112 or other inertcathode, and which would either constitute an n-type or a p-type regionaccording to the amount of cadmium introduced from the anode 116 intosolution. The amount of cadmium introduced via the cadmium anode can becontrolled by adjustment of the two potentiometers 120 and 122.Thereafter, a second layer could be formed on the first-mentioned layerin order to form either a p-type region or an n-type region which isopposite to the first deposited layer. In all cases where two anodes areemployed in the arrangement as illustrated in FIG. 10, or otherwise thearrangement illustrated in FIG. 11 as hereinafter described, the ratioof the metal ion to the non-metal ion or molecule in the compound whichis deposited is determined by the currents flowing through therespective anodes to the single cathode. Moreover, it can also beobserved that it is equally easy to provide semiconductor films with ahomojunction as well as a heterojunction. By merely changing theelectrolyte to form the second layer, it will be possible to form theheterojunction materials.

FIG. 11 illustrates an arrangement whereby a nonmetal anode 126 is usedin place of the cadmium anode 116. Moreover, the electrolyte 124 wouldbe replaced by an electrolyte 128 containing cadmium ions in solution.As indicated, the cadmium could be introduced in the solution, as, forexample, from a solution of cadmium salts. In accordance with thisarrangement, it is possible to carefully control the amount of telluriumintroduced into solution through adjustment of the respectivepotentiometers 120 and 122.

The tellurium anode of FIG. 11 could also be formed as an alloy with,e.g., antimony, phosphorus or arsenic, etc., to provide a dopant. Ineither case, the use of the two anodes provides a means to continuallysupply the minority component in order to slowly replenish the same insolution. Replenishing of the minority component, generally the morenoble component, is usually required when there is a large ratio betweenthe concentrations of the majority and minority components in theelectrolyte In the case where two anodes are not employed, and where alarge ratio does exist, the minority component could be slowly added ona continued basis, as by dripping the same into the electrolyte, basedon the ratio of depletion of the minority component.

In any of these embodiments illustrated in FIGS. 10 and 11, it ispossible to provide the p-type region and n-type region bystoichiometrically adjusting the amount of cadmium with respect to thenon-metals, such as tellurium, selenium, sulfur, etc. Otherwise, it ispossible to introduce dopants into the solutions. In the more preferredform, the dopant could actually be contained within the material formedin one of the anodes as an alloy thereof, as, for example,cadmium-indium alloys as an anode.

With respect to the use of the three or more electrodes, it should beunderstood that plating could occur on one of the electrodes, which maynot constitute a cathode per se. By properly adjusting the components inthe electrolyte and by adjusting the potential applied to theelectrodes, it is actually possible to perform anodic deposition andcathodic deposition at the same time. FIG. 12 illustrates an arrangementwith three electrodes with one of the electrodes 129 being formed of asulfur-containing material and the other of the electrodes 130 beingformed of a cadmium-containing material. A third electrode 132 is alsoprovided and is preferably of an inert material. Again, by mereadjustment of two potentiometers, e.g. the two potentiometers 120 and122, it is possible to carefully regulate the amount of cadmium andsulfur ions which are introduced into solution and which are dischargedat the electrode 132 in order to form a cadmium sulfide film, asillustrated.

While cadmium sulfide has been described herein as an example, any ofthe other metal and non-metal components could be used. Moreover, inthis embodiment, the electrodes cannot be defined as cathodes and anodesin the classical sense. By way of example, the electrode 129 could have,e.g., a negative 2-volt potential, the electrode 132 could have, e.g., apositive 2-volt potential, and the electrode 130 could have, e.g., apositive 4-volt potential. In this way, cadmium from the electrode 130would be discharged and plate out on the electrode 132 through acathodic process and sulfur from the electrode 129 would be dischargedand plate out on the electrode 132 through an anodic process.

As used herein, the terms "inert" or "relatively inert," as, forexample, with an "inert anode" or "inert cathode," refer to a materialwhich is inert with respect to the reactants being employed. Thus, inthe case of an inert cathode, such as a nickel cathode, the cathodewould be inert and nonreactive with respect to the electrolyte or any ofthe ions introduced therein in order to form the semiconductor film onthe cathode.

The present invention is highly effective in obtaining relatively thinfilms by use of the electrochemical techniques described herein. In thisinstance, films with a thickness ranging from about 0.1 micron to about40 microns and larger can be obtained as described above. Thus, the useof the term "thin" or "relatively thin" with respect to the filmthickness will be based on film thicknesses within the range of about0.1 micron to about 40 microns, or perhaps greater.

While the present invention is effective with those materials describedabove, and which can be made in accordance with the processes of thepresent invention, one of the most effective materials thus found foruse in the production of the photo-voltaic cells is that formed ofcadmium telluride. It has been found that photo-voltaic cells based onp-n homojunctions have an expected energy conversion efficiency that isa function of the band-gap of the material used with the optimumband-gap occurring near approximately 1.5 eV. Moreover, it has beenfound that cadmium telluride provides a band-gap in this range. Inaddition, the cadmium telluride provides a reasonably high efficiencyand also a lower cost with respect to other materials which might beemployed. Cadmium telluride is also stable in air, is nontoxic and canwithstand temperature variations of several hundred degrees above andbelow ambient temperatures without decomposing. Moreover, cadmiumtelluride is preferred inasmuch as it is neither deliquescent orhygroscopic, and furthermore, is not subject to disproportionation underconditions of expected terrestrial operation.

FIG. 13 illustrates the steps employed in the method of producing thephoto-voltaic power cells in accordance with the present invention.These steps were actually delineated in connection with the previousdescription. However, referring to FIG. 13, it can be observed that ametal cathode is introduced into the electrolyte and an anode isintroduced into the electrolyte. The method includes the formation ofmolecules or ions of the non-metallic component in the electrolyte andthe formation of ions of the metallic component in the electrolyte. Asindicated previously, these ions could be introduced into solution inseveral different ways, and the ions of both ccomponents would bedischarged at the cathode during the application of the electric field.Both ions and molecules can migrate to the cathode, and upon applicationof the electric field they are discharged and form a coating in the formof a compound semiconductor, as previously described. As indicatedabove, the coating would be first formed with a first region such as ann-type region or a p-type region. The coating would then be providedwith a second region which is the opposite of the first region.

Finally, in the making of the photo-voltaic power cells, conductiveterminals could be applied to both the n-region and the p-region.Otherwise, these terminals could be applied to layers in contact withthe n-region and the p-region in connection with the embodimentsillustrated in FIGS. 3 and 4 of the drawings.

One of the unique results which can be obtained in accordance with thepresent invention is that either a homojunction or a heterojunction canbe established between the p-type region and the n-type region. In thisway, problems of material waste and impurities are substantiallyreduced, and almost completely eliminated. Furthermore, all of theheretofore required stringent control procedures used in the formationof photo-voltaic cells and similar semiconductor materials can becompletely obviated.

Another one of the unique aspects of the present invention is that thereactions heretofore described may be carried out at or close to roomtemperature. Moreover, and as indicated, the processes described hereinresult in very little, if any, waste material. In addition, theprocesses can be carried out with very little concentrations of therequired ions.

The configuration and method of fabrication of the photo-voltaic powergenerating means in accordance with the present invention lendthemselves to continuous processes of production of substantial lengthsand areas of such power generating means. In many applications, powergenerating means may be wound upon a roller or other storage means andsimply laid out as a sheet or surface covering areas exposed to light,such as roofs and walls exposed to solar radiation.

Thus, there has been illustrated and described novel photo-voltaic powergenerating means and methods of use and methods of fabricating suchpower generating means with a relatively high degree of efficiency andwhich fulfill all of the objects and advantages sought therefor. Manychanges, modifications, variations and other uses and applications ofthe power generating means and methods described herein will becomeapparent to those skilled in the art after considering thisspecification and the accompanying drawings. All such changes,modifications, variations and other uses and applications which do notdepart from the spirit and scope of the invention are deemed to becovered by the invention which is limited only by the following claime.

What is claimed is:
 1. A method of preparing a photo-voltaic powergenerating cell comprising:depositing electrochemically on a suitablyprepared metal electrode adapted to form a Schottky barrier a coating ofa semiconductor compound from an electrolytic bath including thecomponents of said semiconductor compound, said compound being capableof forming a Schottky barrier with said electrode, being transmissive tolight radiation and being capable of forming electron-hole pairs uponbeing irradiated with photons, said components being formed of at leastone of the metal elements of Class IIB and non-metal elements of ClassVIA of the Periodic Table of Elements.
 2. A method as defined in claim 1wherein said metal electrode is nickel and said semiconductor compoundis cadmium sulfide.
 3. In a process for depositing a photo-voltaic cellon an electrically conductive semiconductive substrate of a firstconductivity type, the steps of:immersing the substrate into anelectrolytic bath including an acid solution of cadmium sulfate;providing an anode having at least a surface layer of tellurium;applying a voltage between the anode and the substrate forming acathode, the voltage being negative between the cathode and a calomelreference electrode; and continuing the plating process until a firstthin polycrystalline cadmium telluride layer of said first conductivitytype is deposited.
 4. In the process defined in claim 3 wherein thesubstrate is of the n-type and consists of indium tin oxide, and whereinthe first conductivity type is the n-type.
 5. In the process defined inclaim 4 wherein the electrolytic bath includes an n-type donor impurityfor codeposition with the n-type cadmium telluride.
 6. In the processdefined in claim 5 wherein the n-type donor impurity consists of indiumsulfate.
 7. In the process defined in claim 4 which includes theadditional steps of:removing the n-type cadmium telluride layer from theinfluence of the electric voltage whereby a thin p-type layer is formed;subjecting the thus formed p-type layer to the influence of a p-typeacceptor in the electrolytic bath without applying a voltage thereto;and electroplating a thin tellurium layer onto said p-type layer from anelectrolyte including sodium sulfate without any doping impurities byapplying a negative voltage for a time sufficient to create a telluriumlayer having a predetermined thickness at an acid pH.
 8. In the processdefined in claim 7 wherein the p-type acceptor for the formation of thep-type layer consists of arsenic pentoxide.
 9. In the process defined inclaim 3 wherein the substrate is of the p-type and consists of antimonydoped tin oxide, and wherein the first conductivity type is the p-type.10. In the process defined in claim 9 wherein the electrolytic bathincludes a p-type acceptor impurity for codeposition with the p-typecadmium telluride.
 11. In the process defined in claim 3 which includesthe following additional steps:providing an additional electrolyteincluding cadmium sulfate and an active impurity of the oppositeconductivity type; immersing the substrate and the first cadmiumtelluride layer in the additional electrolyte; and applying a negativevoltage with respect to a calomel reference electrode between thecathode and anode, the applied voltage being different from thatutilized for depositing the first cadmium telluride layer, thereby todeposit a second thin polycrystalline cadmium telluride layer of theopposite conductivity type, whereby an n-p junction is formed betweenthe first and second cadmium telluride layers to provide a homojunctiondevice.
 12. In the process defined in claim 11 wherein the substrate isof the n-type and consists of indium tin oxide and wherein the firstconductivity type is the n-type, and wherein the additional electrolyteincludes a p-type acceptor impurity and wherein the second cadmiumtelluride layer is of the p-type.
 13. In the process defined in claim 12wherein the plating voltage for the first cadmium telluride layerbetween the cathode and the calomel reference electrode is more negativethan that applied between the cathode and the calomel referenceelectrode for the second cadmium telluride layer, whereby more telluriumand less cadmium is deposited for the second layer than for the firstlayer.
 14. The process as defined in claim 12 wherein the p-typeacceptor impurity consists of arsenic pentoxide.
 15. The process ofmanufacturing an n-p heterojunction photo-voltaic device utilizing asemiconductive substrate of a first conductivity type, comprising thesteps of:immersing the substrate into an electrolyte comprising cadmiumsulfate at an acid pH; providing an anode having at least a telluriumcoating; applying a voltage between the cathode formed by the substrateand the anode, the voltage being negative between the cathode and acalomel reference electrode so as to deposit an opposite conductivitytype thin polycrystalline cadmium telluride layer; and continuing theplating until the layer has a predetermined thickness, whereby an n-pheterojunction is provided between the substrate of a first conductivitytype and the plated cadmium tellurium layer of the opposite conductivitytype.
 16. The process defined in claim 15 wherein the substrate is ofthe n-type and consists of indium tin oxide and wherein the cadmiumtelluride layer is of the p-type and is obtained by depositingrelatively more tellurium than cadmium.
 17. The process defined in claim16 wherein the electrolyte additionally contains a p-type acceptorimpurity for codeposition with the cadmium telluride layer.
 18. Theprocess defined in claim 17 wherein said p-type acceptor impurityconsists of arsenic pentoxide.
 19. The process defined in claim 16wherein the negative deposition voltage between the cathode and thecalomel reference electrode is so selected with respect to the pH, thata p-type layer of cadmium telluride is formed having more telluride thancadmium.
 20. The process defined in claim 15 wherein the substrate is ofthe p-type and consists of antimony doped tin oxide, and wherein theopposite conductivity type is the n-type, the n-type cadmium telluridelayer being formed by applying a negative deposition voltage between thecathode and the calomel reference electrode so selected that an n-typecadmium telluride layer is formed having less telluride than cadmium.21. The process defined in claim 20 wherein the electrolyte additionallyincludes an n-type donor impurity.
 22. The process defined in claim 21wherein the n-type donor impurity consists of indium sulfate.
 23. Aprocess for manufacturing a Schottky barrier photo-voltaic devicecomprising the steps of:immersing a metal substrate into an electrolyticbath including cadmium sulfate and serving as a cathode; providing ananode in the bath having at least an outer coating of tellurium;maintaining the electrolytic bath at an acid pH; and applying a voltagebetween the cathode and anode, a negative voltage being measured betweenthe cathode and a calomel reference electrode, the voltage being soselected with respect to the pH that the deposited cadmium tellurideconsists of a predetermined ratio of cadmium to tellurium to deposit apredetermined conductivity type layer, the deposition being continueduntil a thin polycrystalline cadmium telluride layer of predeterminedthickness is deposited, said metal substrate being capable of forming aSchottky barrier with the deposited cadmium telluride.
 24. The processas defined in claim 23 wherein the cadmium telluride layer is of then-type and is obtained by deposition more cadmium than tellurium. 25.The process as defined in claim 24 wherein an n-type donor impurity isintroduced into the electrolytic bath for codeposition with the cadmiumtelluride.
 26. The process defined in claim 25 wherein the n-type donorimpurity consists of indium sulfate.
 27. The process defined in claim 23wherein the deposited cadmium telluride layer is of the p-type andconsists of less cadmium than tellurium.
 28. The process defined inclaim 27 wherein a p-type acceptor impurity is introduced into theelectrolytic bath for codeposition with the cadmium telluride.
 29. Theprocess defined in claim 28 wherein the p-type acceptor impurityconsists of arsenic pentoxide.
 30. The process for forming aphoto-voltaic cell on an electrically conductive n-type semiconductivesubstrate consisting of indium tin oxide, the process comprising thesteps of:immersing the substrate into an electrolytic bath including anacid solution of cadmium sulfate; providing an anode having at least asurface layer of tellurium; applying a plating voltage between the anodeand the substrate forming the cathode, the voltage being negativebetween the cathode and a calomel reference electrode; continuing theplating process until a thin polycrystalline cadmium telluride layer ofn-type conductivity is deposited; immersing the n-type cadmium telluridelayer into an electrolytic bath including an acid solution of cadmiumsulfate and sodium sulfate; converting the surface of the cadmiumtelluride layer into a p-type cadmium telluride layer; andelectroplating a thin tellurium layer onto said p-type cadmium telluridelayer for a predetermined period of time to create a tellurium layer ofpredetermined thickness.
 31. The process defined in claim 30 wherein ap-type acceptor impurity is added to the electrolytic bath for platingthe p-type cadmium telluride layer.
 32. The process defined in claim 31wherein the p-type acceptor impurity consists of arsenic pentoxide.